[97] At hypersonic
speeds, the turbojet engine gives way to a much simpler air-breathing
engine: the ramjet. The high-speed compressors and turbines of the
turbojet are forgotten in the ramjet because the highvelocity air
scooped up by the engine intake reaches

[98] pressures sufficient
for engine operation as it "rams'' into the combustion chamber.
Langley Research Center, in association with the Garrett Corporation,
designed a hypersonic ramjet research engine for inflight testing on
the X-15 rocket plane at Mach 6. This engine, however, was strictly
experimental and incorporated several untested features, such as the
use of hydrogen fuel in a combustion chamber operating at about
5000° F. Rather than risk a pilot by mounting this veritable
bomb on the X-15, NASA asked Lewis Research Center to test the engine
on the ground first. In 1970 NASA-Lewis already had the key elements
for a large blowdown hypersonic tunnel in place at its Plum Brook
station on Lake Erie, 50 miles west of Cleveland. The relative
remoteness of the Plum Brook facility made it an ideal place to test
what was bound to be a very noisy engine. For heat transfer research,
a large 5000-psi tank farm had been installed there plus an
induction-heated, graphite pebble-bed heater capable of raising a
test gas to 3 500 ° F. To solve the chronic problem of oxidation
of the graphite heater at extreme operating temperatures, inert
gaseous nitrogen was initially passed through the heater. The
controlled addition of....

A hypersonic tunnel was built at
Lewis Research Center to test a fuII-scale ramjet engine. This
blowdown tunnel operated between Mach 5 and Mach 7 with a
hydrogen-burning ramjet under test.

[99] oxygen to the
nitrogen stream downstream of the heater but ahead of the test
section provided a test medium that matched the constituents of
atmospheric air in supporting combustion.

To convert the heat-transfer equipment into a
hypersonic tunnel, Lewis personnel added three 42- inch water-cooled
nozzles sized for Mach 5, 6, and 7 operation. A large steam ejector
served to reduce tunnel pressures to those typical of high
altitudes.

A full-scale "boilerplate" hypersonic engine
was installed in the Plum Brook tunnel. Aerodynamically it conformed
to the basic design, but there were no restrictions on structure
weight for the ground tests. The engine did operate properly, burning
hydrogen fuel successfully, but the thrust levels were lower than
anticipated. Nevertheless, the tests were considered successful,
auguring well for the eventual construction of a flight-model
hypersonic ramjet and Mach 6 operation with air-breathing rather than
rocket engines.

Full-scale "boilerplate" version of
a hypersonic ramjet mounted in the Lewis hypersonic
tunnel.